1. Field of the Invention
The present invention relates to a method for confirming the accuracy of communications performed in a signal transmit-receive device using optical or electrical signals. More particularly, it relates to a method of loopback tests run between a communication device and a signal transmit-receive device.
2. Description of the Related Art
Conventionally, an optical or electrical signal transmit-receive device in which a transmitting circuit and a receiving circuit are paired have been used in an exchange, router and like used for signal communications. In such a signal transmit-receive device, to test the accuracy of communications transmissions, a test signal is looped from a communication device through a transmitting circuit and a receiving circuit and back to the communication device without processing. The communication device compares a second, transmitted test signal with the test signal looped back from the signal transmit-receive device to determine the accuracy of transmitted communications signal. This test method is referred to as the loopback test method. A method of the prior art is disclosed in, e.g., U.S. Pat. No. 5,787,114.
Referring to FIG. 10, the problems of the conventional loopback method are described. In FIG. 10, a communication device 1001 transmits and receives parallel signals, a communication device 1003 transmits and receives serial signals, and a signal transmit-receive device 1002 performs mutual conversion of parallel signals and serial signals between the two communication devices to transmit the signals. A transmitting circuit 1004 and a receiving circuit 1008 are formed on ICs different from each other. A serializer 1005 converts parallel signals into serial signals, a transmitting driver 1007 drives a serial signal TX, an input buffer 1011 receives a serial signal RX, and a deserializer 1009 converts serial signals into parallel signals. In the signal transmit-receive device, during normal communication, a parallel signal TXD transmitted from the communication device 1001 is converted into a serial signal TX, which is transmitted to the communication device 1003. Conversely, a serial signal RX transmitted from the communication device 1003 is converted into a parallel signal RXD, which is transmitted to the communication device 1001. During a loopback test, a test signal transmitted from the communication device 1001 is converted by the serializer 1005 into an output signal TX, which is looped back to the communication device 1001 without processing, through the wiring 1006 for connecting the transmitting circuit 1004 and the receiving circuit 1008, and the deserializer 1009.
However, this conventional method requires that a signal of the same quantity as the test signal be fed between a transmitting circuit group and a receiving circuit group. This requires a circuit configuration for signal transfer and transfer time. Particularly, where the signal transmit-receive device must be constituted by different modules or ICs for a transmitting circuit group and a receiving circuit group, a circuit having the same transmission speed as a data signal for connecting the transmitting circuit group and the receiving circuit group is required to loop back a signal. Recent data signals generally have a high transmission speed over 1 Gbps, and high-speed signal lines and integrated circuit (IC) pins are required for connection between the transmitting circuit group and the receiving circuit group. However, high-speed signal lines and IC pins are generally more expensive than low-speed signal lines and IC pins, inviting an increase in installation costs. Also, an increased number of signal lines requires additional input-output circuits and invites an increase in power consumption. Use of inexpensive signal lines and IC pins is not feasible, however, since signal transmission rates would be unacceptably slow thereby degrading device performance.